An initialization argument controls object creation and
initialization. It is often convenient to use keyword symbols to name
initialization arguments, but the name of an initialization argument
can be any symbol, including nil. An initialization argument
can be used in two ways: to fill a slot with a value or to provide an
argument for an initialization method. A single initialization
argument can be used for both purposes.

An initialization argument list is a list of alternating
initialization argument names and values. Its structure is identical
to a property list and also to the portion of an argument list
processed for &key parameters. As in those lists, if an
initialization argument name appears more than once in an
initialization argument list, the leftmost occurrence supplies the
value and the remaining occurrences are ignored. The arguments to
make-instance (after the first argument) form an initialization
argument list. Error checking of initialization argument names is
disabled if the keyword argument pair whose keyword is
:allow-other-keys and whose value is non-nil appears in the
initialization argument list.

An initialization argument can be associated with a slot. If the
initialization argument has a value in the initialization argument
list, the value is stored into the slot of the newly created object,
overriding any :initform form associated with the slot. A
single initialization argument can initialize more than one slot. An
initialization argument that initializes a shared slot stores its
value into the shared slot, replacing any previous value.

An initialization argument can be associated with a method. When an
object is created and a particular initialization argument is
supplied, the generic functions initialize-instance,
shared-initialize, and allocate-instance are called with that
initialization argument's name and value as a keyword argument pair.
If a value for the initialization argument is not supplied in the
initialization argument list, the method's lambda-list supplies a
default value.

Initialization arguments are used in four situations: when making an
instance, when re-initializing an instance, when updating an instance to
conform to a redefined class, and when updating an instance to conform
to the definition of a different class.

Because initialization arguments are used to control the creation and
initialization of an instance of some particular class, we say that an
initialization argument is ``an initialization argument for'' that
class.